firefox/testing/web-platform/tests/css/css-cascade/scope-shadow.html

<!DOCTYPE html>
<title>@scope - ShadowDOM</title>
<link rel="help" href="https://drafts.csswg.org/css-cascade-6/#scope-atrule">
<link rel="help" href="https://github.com/w3c/csswg-drafts/issues/9025">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>

<div id=host_plain>
  <template shadowrootmode=open>
    <style>
      @scope (:host) {
        .a {
          z-index: 1;
        }
      }
    </style>
    <div class=a>
    </div>
  </template>
</div>
<script>
  test(() => {
    let a = host_plain.shadowRoot.querySelector('.a');
    assert_equals(getComputedStyle(a).zIndex, '1');
  }, '@scope can match :host');
</script>

<div id=host_functional>
  <template shadowrootmode=open>
    <style>
      @scope (:host(div)) {
        .a {
          z-index: 1;
        }
      }
      /* Should not match: */
      @scope (:host(span)) {
        .a {
          z-index: 42;
        }
      }
    </style>
    <div class=a>
    </div>
  </template>
</div>
<script>
  test(() => {
    let a = host_functional.shadowRoot.querySelector('.a');
    assert_equals(getComputedStyle(a).zIndex, '1');
  }, '@scope can match :host(...)');
</script>

<div id=host_scope_subject>
  <template shadowrootmode=open>
    <style>
      @scope (:host) {
        :scope {
          z-index: 1;
        }
      }
    </style>
    <div class=a>
    </div>
  </template>
</div>
<script>
  test(() => {
    assert_equals(getComputedStyle(host_scope_subject).zIndex, '1');
  }, ':scope matches host via the scoping root');
</script>

<div id=host_scope_subject_is>
  <div class=host>
    <template shadowrootmode=open>
      <style>
        /* Should not match host, nor outside shadow. */
        :is(:scope, .a, .host) {
          z-index: 2;
        }

        @scope (:host) {
          :is(:scope, .a) {
            z-index: 1;
          }
        }
      </style>
      <div class=a>
      </div>
    </template>
  </div>
  <div class=a>
  </div>
</div>
<script>
  test(() => {
    let host = host_scope_subject_is.querySelector('.host');
    assert_equals(getComputedStyle(host).zIndex, '1');
    let a = host.shadowRoot.querySelector('.a');
    assert_equals(getComputedStyle(a).zIndex, '1');

    let a_outside = host_scope_subject_is.querySelector('.a');
    assert_equals(getComputedStyle(a_outside).zIndex, 'auto');
  }, ':scope within :is() matches host via the scoping root');
</script>

<div id=implicit_scope_shadow_parent>
  <div class=host>
    <template shadowrootmode=open>
      <style>
          @scope {
            /* Matches host */
            :scope {
              z-index: 1;
            }
            :scope > .a {
              z-index: 2;
            }
          }
      </style>
      <div class=a>
      </div>
    </template>
  </div>
</div>
<script>
  test(() => {
    let host = implicit_scope_shadow_parent.querySelector('.host');
    let a = host.shadowRoot.querySelector('.a');
    assert_equals(getComputedStyle(host).zIndex, '1');
    assert_equals(getComputedStyle(a).zIndex, '2');
  }, 'Implicit @scope as direct child of shadow root');
</script>

<div id=implicit_scope_constructed>
  <div class=host>
    <template shadowrootmode=open>
      <div class=a>
      </div>
    </template>
  </div>
</div>
<script>
  test(() => {
    let host = implicit_scope_constructed.querySelector('.host');
    let sheet = new CSSStyleSheet();
    sheet.replaceSync(`
      @scope {
        :scope {
          z-index: 1;
        }
        :scope .a {
          z-index: 2;
        }
    `);
    host.shadowRoot.adoptedStyleSheets = [sheet];
    let a = host.shadowRoot.querySelector('.a');
    assert_equals(getComputedStyle(host).zIndex, '1');
    assert_equals(getComputedStyle(a).zIndex, '2');
  }, 'Implicit @scope in construted stylesheet');
</script>

<div id=scope_via_parent_pseudo_subject>
  <div class=host>
    <template shadowrootmode=open>
      <style>
          @scope (:host) {
            :scope {
              & {
                z-index: 1;
              }
            }
          }
      </style>
    </template>
  </div>
</div>
<script>
  test(() => {
    let host = scope_via_parent_pseudo_subject.querySelector('.host');
    assert_equals(getComputedStyle(host).zIndex, '1');
  }, 'Matching :host via &, :scope (subject)');
</script>

<div id=scope_via_parent_pseudo_non_subject>
  <div class=host>
    <template shadowrootmode=open>
      <style>
          @scope (:host) {
            :scope .a {
              & {
                z-index: 1;
              }
            }
          }
      </style>
      <div class=a>
      </div>
    </template>
  </div>
</div>
<script>
  test(() => {
    let host = scope_via_parent_pseudo_non_subject.querySelector('.host');
    let a = host.shadowRoot.querySelector('.a');
    assert_equals(getComputedStyle(a).zIndex, '1');
  }, 'Matching :host via &, :scope (non-subject)');
</script>

<div id=scope_via_parent_pseudo_non_subject_child>
  <div class=host>
    <template shadowrootmode=open>
      <style>
          @scope (:host) {
            :scope > .a {
              & {
                z-index: 1;
              }
            }
          }
      </style>
      <div class=a>
      </div>
    </template>
  </div>
</div>
<script>
  test(() => {
    let host = scope_via_parent_pseudo_non_subject_child.querySelector('.host');
    let a = host.shadowRoot.querySelector('.a');
    assert_equals(getComputedStyle(a).zIndex, '1');
  }, 'Matching :host via &, :scope (non-subject, >)');
</script>